Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

HEALTH PHYSICS INSTRUMENTATION 471
Figure 9-30. Diagram of a long
counter, a neutron counter whose
response is approximately uniform from
about 10 keV to 5 MeV. (Reproduced
with permission from Hanson AO,
McKibben JL. A neutron detector having
uniform sensitivity from 10 kev to 3
Mev. Phys Rev. 1947; 72(8):673–677.
Copyright c○ 1947 American physical
Society.)
the counter. The counter is enclosed in a thin sheet of cadmium to absorb thermal
neutrons in order to prevent pulses due to deuteron recoils following absorption
of thermal neutrons by the hydrogen. The hydrogenous material may also be a
solid, such as paraffin or polyethylene, incorporated into the wall of the counter.
The fast neutrons “knock out” protons from these solids, and the protons dissipate
their energy in the counter gas. When used in this way as a source of protons, the
hydrogenous substance is called a proton radiator.
The sensitivity for fast neutrons of a proton recoil counter is very much less than
the sensitivity of a BF3 counter for thermal neutrons. This is true for two reasons:
The cross section of hydrogen for scattering of fast neutrons is very much less than
the slow-neutron capture cross section of 10 B, and also the energy distribution of the
scattered protons includes a large fraction of very low energy protons. For neutron
energies up to about 10 MeV, the scattering of neutrons is isotropic. This means that
the energy of the scattered proton may vary from zero to the energy of the neutron.
The pulses that result from protons to which little energy was imparted during the collision
are therefore not counted because of the bias against gamma radiation. Above
this threshold, the energy response of the recoil proton proportional counter is determined
mainly by the energy dependence of the scattering cross section of hydrogen.
Neutron Dosimetry
The dose equivalent from neutrons depends strongly on the energy of the neutrons,
as shown in Table 9-5. We therefore cannot simply convert neutron flux density
(fluence) into dose equivalent unless we know the energy spectral distribution of
the neutrons. For thermal neutrons, of course, the spectral distribution is known, and
aBF3 counter can be calibrated to read directly in millirads per hour. For higher
energy neutrons, neither the moderated BF3 long counter with its “flat” energyindependent
response to fast neutrons nor the simple proton-recoil proportional
counter, whose response to fast neutrons depends strongly on the scattering cross
section, is suitable for dosimetry.